1. Field of the Invention
The present invention relates to a synchronizer to be incorporated into a receiver of a spread spectrum communication apparatus used in digital mobile radio communication.
2. Description of the Related Art
The conventional spread spectrum communication apparatus synchronizes with sampling data which is received and subjected to A/D conversion, searches data having the optimum phase and despreads the data having the optimum phase to demodulate the received data. For instance, a synchronizer of such a conventional spread spectrum communication apparatus is shown in FIGS. 9 and 10. FIG. 9 is a block diagram illustrating the configuration of a receiver synchronizer of the conventional spread spectrum communication apparatus, and FIG. 10 is a detailed block diagram of a search unit of the receiver synchronizer of the conventional spread spectrum communication apparatus illustrated in FIG. 9.
First, referring to FIG. 9, the configuration of the receiver synchronizer of the conventional spread spectrum communication apparatus will be described. In FIG. 9, reference numeral 901 designates an A/D converter for converting a received signal from an analog signal to a digital signal; 902, a DLL (Delay Lock Loop) for time tracking a PN (pseudonoise) signal so as to be in phase with it while sampling data output from the A/D converter is correlated with the PN signal; 903, a search unit for searching the data received through a path from which the maximum power is obtained (a path over which the received signal flows from a transmitter to a receiver or a signal received through this path), using the correlation result calculated from an output of a correlation device; 904, a sampling clock signal generator controlled by the DLL; and 905, a data demodulation correlation device for despreading the received data.
With reference to FIG. 10, the configuration of a receiver synchronizer of the conventional spread spectrum communication apparatus illustrated in FIG. 9 will be described. In FIG. 10, reference numeral 1001 designates a search PN signal generator for generating PN signals having all PN phases (phases of the pseudonoise signals); 1002, a search correlation unit for correlating the sampling data with the PN signal to output correlation data corresponding to the correlation; 1005, a data buffer for storing the correlation data output from the search correlation unit 1002; 1000, a path search unit which controls the search PN signal generator so as to search to select for the correlation data corresponding the maximum-power path from the data buffer 1005 and outputs the PN phase of the thus-searched correlation data; and 1003, a PN signal generator for outputting a PN signal to the DLL 902.
With reference to FIGS. 9 and 10, the operation of the receiver synchronizer of the conventional spread spectrum communication apparatus will be described. First, the DLL 902 is placed into an inoperative state, and the timing of the operating sampling clock signal generator 904, which controls the sampling timing of the A/D converter 901, is made constant. A received signal is sampled at predetermined timing by the A/D converter 901, and the thus-sampled received signal is input into the search unit 903.
The search correlation unit 1001 correlates the sampling data input to the search unit 903 with the PN signal output from the search PN signal generator 1001. The resultant correlated data is stored in the data buffer 1005. When the search results regarding all the PN phases is stored in the data buffer 1005, the correlation data obtained from the maximum-power path is searched to be selected from all data stored in the buffer (a searching operation).
A PN phase with respect to the maximum-power path is output to the PN signal generator 1003, and the DLL 902 is activated by the PN phase, thereby commencing a tracking operation (a tracking operation). Further, the PN signal output from the PN signal generator 1003 is output to the data demodulation correlation device 905, thereby commencing demodulation of data. After completion of the searching operation, the phase of the PN signal generator 1003 remains unchanged, and the time tracking of the path is carried out by use of the PN phase of the PN signal generator which is set at the time of completion of the searching operation (a synchronous period).
As described above, the receiver synchronizer of the conventional spread spectrum is arranged so as to synchronously track the path by operating only the DLL 902 without altering the phase of the PN signal generator 1003 after the completion of the searching operation. Accordingly, if the level of the path tracked by the DLL 902 is significantly reduced for reasons of e.g., fading, the DLL 902 comes out of synchronization with the path. If the DLL 902 comes out of synchronization with the path, it is impossible to demodulate the data. In order to demodulate the data, all the operations must be carried out once again from the first searching operation.